Shielded-metal arc welding (SMAW) is a versatile welding process used for forming high quality welds. For example, in gas and chemical piping applications, the welds must be of the highest quality because a failed weld could be catastrophic. The quality or integrity of the welds are generally tested by X-rays or ultrasonic inspection equipment, which will reveal defects in the welds.
Shielded-metal arc welding is also a simple process in principle requiring only a high current source, insulated electrical cables, and an electrode holder for holding a consumable electrode, which is used to supply additional metal to form a continuous weld. Welding electrodes are also called welding "rods" or "wires". The high current source is supplied by a welding machine, which is basically an engine-powered electric generator.
A welding electrode for use in SMAW typically consists of a metal core surrounded by a flux covering. In the welding process, an electric arc is formed between the flux-covered metal electrode and the metal being welded. Particularly, the electric arc is generated by touching the tip of a coated electrode to the workpiece and withdrawing it an appropriate distance to maintain the arc. The heat generated melts a portion of the electrode tip, its coating, and the base metal in the immediate area. The electrode, as it moves down the length of the weld, is consumed. The molten electrode and the base metal must be shielded against the ambient atmosphere, which includes oxygen and nitrogen, which may interact with the molten metal and cause voids, porosity, and other weld defects. The shielding is supplied by the flux coating of the electrode. As the flux coating is consumed, it creates a gas shield which protects the weld from oxygen and nitrogen during the welding process and also forms a solid protective slag, which protects the weld during cooling, which must later be chipped away.
It is clear from the above discussion that the welding electrode with its flux coating is the most important element of the SMAW process. The composition and condition of the welding electrodes contribute to the quality of the welds. For high quality welding work, welding electrodes are composed generally of a high tensile strength steel core and an outer coating of low-hydrogen material, which may include iron and carbon powder. These "low-hydrogen" electrodes avoid the problems associated with defective welds, such as voids, porosity, lack of fusion, and slag inclusions, which may cause a weld to fail an X-ray inspection, for example. The low-hydrogen electrodes are therefore preferable for high-quality welding work. Electrodes of this type are commonly available from welding supply companies such as Lincoln, Hobart, and Merriam-Graves. Low-hydrogen electrodes are identified as 70-series electrodes, and include different grades and tensile strengths, identified by American Welding Society classifications 7018 through 7024.
Prior to use in the welding process, low-hydrogen electrodes must be properly stored and handled to prevent the absorption of atmospheric moisture which would degrade the low-hydrogen coating on the rods and result in poor quality welds. Moisture is eliminated by keeping the welding electrodes at an elevated temperature within a specified temperature range immediately prior to their use. Welding electrodes are typically sold in boxes that have a moisture proof packaging. These sealed boxes are often carried separately to the work-site, where the welding process will be performed. When the packaging is opened, the moisture-proofing is breached, and the welding electrodes must be transferred to a holding oven as soon as possible, to avoid moisture absorption by the welding electrode coating.
The problem is compounded because high quality welding work often occurs at remote locations, which is particularly true for gas and chemical piping work, for example. Because welding occurs in remote locations, welding machines include gasoline, diesel, or propane powered engines to generate electricity used in the arc welding process.
Portable electrical holding ovens are known for the storage of a small number of welding electrodes at the job site. These holding ovens operate exclusively on electric current. A number of companies manufacture electrode ovens such as Henkel Incorporated of Hammond, La.
There are two drawbacks to the presently available holding ovens. First, the known holding ovens do not hold a large number of electrodes, partly because the electric power requirements for heating a large quantity of welding electrodes would be excessive. Secondly, all known holding ovens are electrically-powered. At a remote welding site in particular, electric power outlets are generally not available. Welding machines typically include a plurality of electrical outlets for providing power to operate power tools, for example. However, portable electric holding ovens are often plugged into the welding machine outlets as the only available source of electricity at the remote site. A portable electric holding oven, when powered from the welding machine, puts a high electrical load on the welding machine, causing the welding machine engine to operate constantly at fast idle. This causes a greatly increased ambient noise level for the welding operator and excessive fuel consumption for the welding machine.
The present invention overcomes the disadvantages of the prior portable holding ovens, such as the requirement for electric power at remote locations.